When a developing device of an image forming apparatus is driven, toner that is not sufficiently charged is scattered around a photoconductor and a magnet roller. A duct is attached to the container of the developing device. Alternatively, a duct is formed in the container of the developing device by the wall surface of the container.
One end of the duct is opened into the container. At the other end of the duct, a fan for sucking scattered toner is provided. The fan is driven by a fan motor.
The fan motor is controlled by a central processing unit (CPU) via a motor driver such as an integrated circuit (IC).
Conventionally, an image forming apparatus is proposed which sucks scattered toner and thereby prevents contamination due to the toner in and out of the apparatus (JP-A-2002-268482).
JP-A-2002-268482 discloses that a fan of a flow path unit which serves as an air flow path is actuated synchronously with a development driving system motor interlocked with a developing device unit.
Both a photoconductive drum and a magneto roller of a developing device receive a rotational driving force from one main motor via a gear and so on. The shaft, gear and belt of the main motor constitute a driving force transmission mechanism that provides the driving force of the main motor to the photoconductive drum.
The main motor is controlled by the CPU via another motor driver which is different from the motor driver of the fan motor.
A developer stirring member that stirs toner is provided in the developing device. The developer stirring member is a mixer. The mixer has a shaft and a blade member for stirring toner.
The developer stirring member receives a rotational driving force from the main motor via the driving force transmission mechanism. When the magnet roller rotates, the developer stirring member rotates together with the magnet roller.
A toner cartridge is attached to the developing device via a toner replenishment path. A toner replenishment mechanism that supplies toner is provided in the toner cartridge. The toner replenishment mechanism is driven by a toner replenishment motor via the driving force transmission mechanism.
Conventionally, in the image forming apparatus, the photoconductive drum, the magnet roller and the developer stirring member share the main motor.
During printing, the CPU causes all of the photoconductive drum, the magnet roller and the developer stirring member to operate. During the period after one sheet of paper is carried and before the next sheet is carried, the CPU instructs the motor driver of the main motor to rotate the main motor.
As printing is finished, the CPU instructs the motor driver of the main motor to stop rotation of the photoconductive drum, the magnet roller and the developer stirring member.
During the period when a print job is not executed after processing to carry a sheet is finished, the CPU carries out forced replenishment of the developing device with a developer. During the period after the previous print job is finished and before the next print job starts, the CPU causes the toner replenishment mechanism to carry out follow-up toner replenishment.
The CPU is enabled to read concentration information from a toner concentration sensor provided within the developing device. If the CPU determines that the toner concentration is low, the CPU changes the operation mode of the developing device to a forced toner replenishment mode. Also in the forced toner replenishment mode, the CPU causes the toner replenishment mechanism to carry out toner replenishment.
In the image forming apparatus according to the conventional example, the CPU controls the motor driver of the fan motor and the motor driver of the main motor so that on and off timing of the operation of the fan as a scattered toner suction member coincides with on and off timing of the operation of the driving force transmission mechanism.
After driving of the photoconductive drum is stopped, the photoconductive drum continues rotating for a predetermined time because of inertia. Since the photoconductive drum is a component to be replaced periodically, the time for which the photoconductive drum continues rotating should be minimized.
Recently, in the electrographic printing device, the CPU is required to carry out processing to minimize the idling time of a periodically replaced component, when operating in the forced toner replenishment-mode. The CPU drives the developer stirring member while keeping the photoconductive drum and the magnet roller in non-operating state.
For example, CPU controls a one-way clutch in the driving force transmission mechanism and thereby causes only the developer stirring member to operate, of the photoconductive drum, the magnet roller and the developer stirring member.
In this case, the CPU gives an instruction to the motor driver of the main motor so that the operation of the photoconductive drum becomes off, the operation of the magnet roller becomes off and the operation of the developer stirring member becomes on. The CPU also gives a control instruction to the motor driver of the fan motor so that the operation of the fan motor becomes on.
The on and off timing of the operation of the fan is the same as the on and off timing of the operation of the driving force transmission mechanism. During printing, the photoconductive drum is rotated by the main motor and therefore also the developer stirring member is rotated by the main motor.
However, in the forced toner replenishment mode, the CPU cannot rotate the developer stirring member alone.
Consequently, scattered toner generated in the developing device due to the rotation of the developer stirring member cannot be sucked through the duct.